The keypad (up/down/left/right/enter/mode) is used to navigate the player's mp3 collection (stored, in my case, on an NFS network share). I've created a repository on github which includes all the code (both on the AVR and router-side), all the Eagle schematics and brd files and a description of how the player should be used.

If you are familiar with git, this should be all you need to get started.